Questions tagged [thermodynamics]

Covers the study of (mostly homogeneous) macroscopic systems from a heat/energy/entropy point of view. Consider also using tag: [statistical-mechanics].

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12 views

Looking for the efficiency of an adiabatic–isobaric–isochoric thermodynamic cycle

I am trying to prove that the efficiency of the above cycle is equal to: $e = 1 - \gamma \frac{V_2 - V_1}{P_1 - P_2}$ for 1 mole of an ideal gas. I assume I should use the equation $e \equiv \frac{\...
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200 views

Temperature change of water and the copper can

The question is: "0.38kg of a liquid at 12 degrees C is heated in a copper can weighing 0.9kg by an electrical heater of power 20W for 3 minutes. It reaches a temperature of 17C. Assuming no heat loss ...
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3answers
112 views

Prevent aluminium vapour condensing on a surface

I have a situation where aluminium vapour is created in a vacuum chamber ($10^{-6} $ mbar). I would like a window to look into the chamber, however the aluminium vapour produced condenses on the ...
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2answers
21 views

Calculate power of a solar heater

This week-end I build a small solar heater of 1.2meter by 1.2meter. Basically a closed box with the back painted in black and front cover with a transparent plastic. I trying to know the rough ...
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Applying the Clausius Inequality to Three Systems

Suppose we have three systems made up of the same amount of the same substance, $S_1, S_2$ and $S_3$. They start with temperatures $T_1$, $T_2$, $T_3$ such that $T_1>T_2>T_3$. We can place the ...
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Does the viscosity of oil increase with rising temperature?

In order to calculate the viscosity index of a lubricant, apparently you need two input parameters. The kinematic viscosity (mm²/s) at 40°C and at 100°C. If I got it right, mm²/s means the distance ...
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3k views

Extracting heat energy from a material

Does it violate any physical laws to take a portion of the energy out of a system and use it? Specifically I'm referring to heat. (Kinetic energy). For example, if you have a material which has a lot ...
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Air conditioning a home with doors of each room open [on hold]

Assuming that all windows and entrance of a $n$-room home are closed, will it take the same energy to cool the entire home with the doors of each room open as having all the doors closed?
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91 views

What is heavier, water or ice?

I was reading this link and I never saw a definite answer. One would think if you have 1000 pounds of water, and you froze it and weight it you would still have the same 1000 pounds of water when ...
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1answer
68 views

Push and pull a piston [closed]

Is the following understanding correct? Consider there is gas in an adiabatic cylinder with a piston. The energy of the gas is $U$. If I push the piston, $U$ increases. After that, if I pull the ...
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29 views

What to use as reference state for free energy curve of a gas?

My goal is to determine the temperature at which xenon would sublimate (if it didn't turn into liquid first). To do this, I set the free energy of solid Xenon equal to the free energy of Xenon gas, $ ...
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180 views

Microscopic explanation of colligative properties from the entropy related viewpoint

In the chapter 28.6 of book "Concepts in thermal physics "(2nd edition) written by Stephen J. Blundell, there is a sentence says "Adding a small quantity of solute to a solvent increases the entropy ...
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27 views

Heat definition in isotherm and first principle of thermodinamic

We know that the heat Q can be computed by $Q=cm\Delta T$ and in a phase change by $Q=Lm$ (L latent heat). In isotherm process and with no change of phase we can't use those formulas and there's no ...
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23 views

Johnson-Nyquist Noise and the Assumption of a Noiseless Load

I'm a little stuck on a subtlety of RF/microwave design that I find a bit baffling. It has to do with the convention of using of noiseless resistors as loads in circuit noise analysis. I have not yet ...
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60 views

A Contradiction in work done or work energy theorem?

Suppose there exists a Force $F$, acting on a body, which is initially having a velocity $v$, in a direction opposite to that of it's motion. Let the point of application of force produce a ...
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Why are solar panels kept tilted?

I have noticed that, in my country India, most of the solar panels are tilted southward at an angle of ${45}^{\circ} .$ Even on buildings with inverted V-shaped roofs, solar panels are still oriented ...
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1answer
40 views

What is the significance of the height of these verical lines in Thermal Conductivity graph?

The following image is from the Wikipedia Article on Thermal Conductivity: Please click here if you want a good image resolution, as I was unable to upload it here due to "Unsupported Image Format". ...
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1answer
264 views

What is the relationship between flow work and velocity of gas?

As we know that for open systems, we have energy conservation equation as $$h+\frac{v^2}{2}+zg=const.$$ $$h=u+pV$$ $$u+pV+\frac{v^2}{2}+zg=const.$$ Here $pV$ is flow energy. We know that all these ...
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28 views

Delta over variable relations

In the text I'm reading, which discuss the heat transfer by convection in stars, it shows that the relation $\rho \propto \frac{P}{T}$ (equation of state for an ideal gas) implies $$\frac{\Delta\rho}{\...
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4answers
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How to work out the relation between the “mean relative speed” and the “mean speed”?

I'm a freshman and am taking the general physics course. I just learned intro thermodynamics. One problem that really puzzles me is the calculation of "collision mean-free path", where calculating the ...
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Are there any known negative heat capacities?

If we take heat capacity to be defined as "the ratio of the heat added to the temperature rise": $$ C=\frac{\text{d} Q_{rev}}{\text{d}\theta}$$ then this leads me to ask: can this ever be negative? ...
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218 views

Voltage and Fermi levels

It is well known that the voltage $V$ across, say, a p-n junction can be related to the electro-chemical potentials (usually and improperly called Fermi levels) at the borders of the junction $$ qV = \...
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238 views

Feynman Lectures Vol I 41-2: why can $\omega$ replace $\omega_{0}$ in deriving Rayleigh’s law?

NB: I understand that the model presented here is the "failing" classical model. I'm just trying to understand the formal reasoning of the model. In The Feynman Lectures on Physics Vol. I Ch. 41-2 ...
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75 views

How to express $k_BT$ as a length scale?

I am wondering how we can use $k_BT$ to define a length scale? I mean it is a unit of energy, not of length. So how would that transformation work?
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Why does a black hole have much entropy?

Is that just an assumption by Bekenstein etc? For example, what is the different states a non-rotating primordial black hole can be in?
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Why should $E$ and $B$ both contribute $(1/2) kT$ in equilibrium radiation?

At high temperatures/low frequency, energy distributes itself by equipartition. There's a factor of $\frac{1}{2}kT$ for each degree of freedom that contributes quadratically to the energy. So, for ...
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19 views

What is the temperature at which water will boil at an altitude where pressure drops by 10%? [on hold]

If there is a 10% decrease in the atmospheric pressure at a hill compared to the pressure at sea level, then the change in the boiling point of water is_____ °C. (Take latent heat of vaporisation of ...
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20 views

How can saturated steam be in vapor-liquid equilibrium if all water has evaporated?

I'm reading Saturated steam on Wikipedia. It is said: ... water vapor that includes water droplets is described as wet steam. If wet steam is heated further, the droplets evaporate, and at a high ...
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51 views

Vaporizing water by piston work

Consider you have an adiabatic container with a piston. At the moment, the the container has liquid water and air, of known mass, in the headspace at 1 bar and 298 K. Suppose I perform some PV work by ...
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2answers
178 views

Derivation of temperature / entropy relation from statistical mechanics

I am trying to prove that \begin{equation} \frac{1}{T} := k_B\beta = \frac{\partial S}{\partial E}. \end{equation} I have that \begin{equation} E := \sum p_i E_i, \end{equation} the Gibbs entropy \...
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How to find volume from pressure, mass flow rate, and burn time?

I must find the minimum volume needed for two tanks, one with liquid oxygen (LOx) and one with liquified natural gas (LNG), knowing only the mass flow rate of each, and the burn time of the engines. ...
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How air pressure depend upon the rate of evaporation? [on hold]

I think these are inversly proportional to each other if not so please explain me
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61 views

Is the kinetic theory of gas relative in nature

In kinetic theory of gas the foremost assumption is that the temperature of an object is due to the kinetic energy of the atoms/molecules contained in it. But as we know that kinetic energy is a ...
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2answers
120 views

How to calculate work for irreversible adiabatic processes?

By the first law, we know $\text{d}U=\delta Q+\delta W$ and, on adiabats, we know $\text{d} U=\delta W$. But what is $\delta W$ for irreversible adiabatic processes? Take a thermally isolated ...
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What is the best way to move air in a room that has several computers? [on hold]

I have a computer room that is $12$' by $20$' in size and holds two large computer systems that generate a lot of heat. The room has a regular $2.5$ ton AC unit and I have a $12,00$ BTU window unit ...
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30 views

Superheated vapor

I have calculated different kind of process temperatures and pressures. I just want to make sure that I have understood this right: Process A: $$p_1=50 \,{bar},T_1=623.15K \to p_2=20 \,{bar}, T_2=...
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2answers
88 views

How are thermonuclear weapons possible?

Given the fact that thermonuclear devices are a mix of fission through enriched uranium whose energy initiates the fusion in surrounding deuterium. Question is (how come) fission raises temperature so ...
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26 views

On source of heat energy dissipation in a system

Suppose we have a block of mass m which was at rest on a rough floor and is now given an impulse towards a wall.It is given that the collision with wall is perfectly elastic and no energy is lost in ...
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53 views

Describing pressure in incompressible fluids

The canonical process of determining the pressure, velocity, and density of a fluid under the influence (or not) of external forces is through simultaneously solving conservation of mass, conservation ...
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1answer
112 views

How is temperature related to quantum vibrational states of molecules?

When a molecule of greenhouse gas absorbs a photon of long wave infrared radiation it is boosted to the next allowed vibrational state. The vibrational state, as I understand it, involves deformations ...
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10 views

Electric cryogenic jet propulsion plausible?

Can air quickly be cooled to a liquid form then expelled from an aircraft as a form of propulsion? Could this liquid air also be fed into a turbine specifically made for the superheating of liquid air?...
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1answer
113 views

Heat transfer direction in fins

I'm unsure of what exactly is changing the heat transfer direction in the following triangular fin: $$ q_{x} = -kA(x)\frac{\mathrm{d}T(x)}{\mathrm{d}x} \tag{1} $$ $$ \begin{align} q_{x+\mathrm{d}x} &...
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39 views

Can the sound change the state of the medium?

The waves perform compression and rarefraccion in the medium where they spread, right?   when the rarefraction happens we should feel the medium in a liquid or gaseous state since the molecules ...
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1answer
188 views

How to formulate surface temperature of a material exposed to sunlight and wind?

I am currently searching for a model or an approach (reference, course, demonstration) to formulate and estimate surface temperature of a known material (for example, concrete, metal), exposed to ...
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1answer
38 views

What quantity remains conserved in a calorimeter when two bodies at different temperatures are mixed?

This is the question: Two identical bodies at different temperatures are mixed in a calorimeter. Which of the following quantities remains conserved? 1) Sum of the temperatures of the two bodies 2) ...
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3answers
94 views

If $\mathrm df$ is an inexact differential, how would the function $f$ look like?

I am studying thermodynamics and in the first chapter the concept of exact and inexact differentials were used to talk about the differences between internal energy, work and heat. From Blundell and ...
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217 views

Reversible engines

As far as my understanding goes, the only reversible heat engine is the Carnot engine, described by the Carnot cycle. On the other hand, in all of the resources I've found the change in entropy for ...
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2answers
119 views

Heat produced in movement of charge from inside of shell to outside

The original question is Two concentric metallic shell’s of radius R and 2R, out of which the inner shell is having charge Q and outer shell is uncharged. If they are connected with a conducting ...
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2answers
447 views

Are isobaric, isochoric, isothermal, adiabatic processes reversible if they all take place quasi-statically?

From my understanding so far, I think that a quasi-static process is one where the system is in thermal equilibrium with itself as well as the surroundings and the process takes place infinitesimally ...